1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly, to a cathode ray tube with uniform brightness and much improved contrast for relieving visual discomfort of viewers.
2. Description of the Related Art
FIG. 1 is a diagram explaining the structure of an already-known color cathode ray tube.
Referring to FIG. 1, the conventional color cathode ray tube includes a front side glass panel 3, and a rear side glass funnel 2 welded to the panel 3. The panel 3 and the funnel 2 are welded to each other in a manner that their interior is under a vacuum, thereby forming a vacuum tube.
A fluorescent screen 7 is formed on the inside surface of the panel 3, and an electron gun 6 is mounted on a neck portion of the funnel 2 opposite to the fluorescent screen 7.
A shadow mask 8 with a color selecting function is located between the fluorescent screen 7 and the electron gun 6, maintaining a predetermined distance from the fluorescent screen 7. The shadow mask 8 is supported by a mask frame 9. The mask frame 9 is elastically supported by a mask spring 1 and connected to a stud pin 4 to be supported to the panel 3.
The mask frame 9 is joined with an inner shield 11 made of a magnetic material to reduce the movement of an electron beam 5 due to an external magnetic field during operation of the cathode ray tube. A deflection yoke 13 for deflecting the electron beam 5 emitted from the electron gun 6 is mounted into a neck portion of the funnel 2. Also, a reinforcing band 10 is included in order to reinforce the front surface glass under the influence of the vacuum state inside the tube.
In operation, the electron beam 5 emitted from the electron gun 6 is deflected vertically and horizontally by the deflection yoke 13, and the deflected electron beam 5 passes through a beam passing hole on the shadow mask 8 and strikes the fluorescent screen 7 on the front, consequently displaying a desired color image. Particularly, the inner shield 11 shields the magnetic field from the rear side of the cathode ray tube.
Whether the panel 3 is explosion proof or has substantially good visibility is heavily dependent on how its inside and outside surface curvatures are formed. In particular, the inside surface curvature has a great impact on the sense of flatness of the screen and the presence of distortion in the image. Further, the transmission rate of the panel 3 plays a very important role for realizing a high quality cathode ray tube because uniform brightness and high contrast are entirely dependent upon the transmission rate.
Generally, the inside surface curvature of the panel can be expressed by a ratio (or wedge) of the thickness of a diagonal end to the thickness at a central portion of the panel (CFT). Compared with a cathode ray tube with a curved outside surface, of which wedge is about 1.30, a cathode ray tube having a substantially flat panel has a wedge greater than 2.2, thus the peripheral portion of this panel, particularly the diagonal end, is extremely thick.
FIG. 2 is a diagram explaining the structure of a panel for the known cathode ray tube.
As illustrated in FIG. 2, the panel 3, which is approximately rectangular in shape, is formed of an effective surface portion 14 where the fluorescent screen is formed, a long side portion 15, a short side portion 16, and a diagonal portion 17. A skirt portion 18 in a curved shape is formed extending away from the edge of the effective surface portion 14 to a rear side of the tube axis direction.
FIG. 3 is a diagram explaining the structure of the effective surface portion of the panel in the known cathode ray tube. Referring to FIG. 3, the substantially flat panel, when the outside effective surface 14 is seen with the naked eye, has an outside surface curvature radius that appears almost flat while the inside surface of the panel has a recognizable curvature. More specifically, the inside surface curvature can be divided into three components: a vertical curvature radius (Rv) in the vertical direction (V), a horizontal curvature radius (Rh) in the horizontal direction (H), and a diagonal curvature radius (Rd) in the diagonal direction. In general, these curvature radii are in a relation of Rd>Rh>Rv. That is, the diagonal curvature radius (Rd) is greater than the horizontal curvature radius (Rh), and the horizontal curvature radius (Rh) is greater than the vertical curvature radius (Rv).
Typically, the wedge, i.e. the ratio of the thickness of a diagonal end (Td) to the thickness at a central portion of the panel (CFT) is in a range of 2.2 to 2.3. As the wedge (Td/CFT) gets closer to 1, the sense of flatness of the screen and manufacturing advantages of the panel are improved. However, it was also discovered that under these conditions the shadow mask at a predetermined distance from the inside surface of the panel 3 could be easily deformed by an external shock.
To obviate such problem, the wedge (Td/CFT) is usually set higher than 2.2.
However, increasing the wedge means decreasing the thickness at the central portion of the panel 3 (CFT) in contrast to the thickness of the diagonal end (Td). In doing so, the panel 3 often breaks down during a thermal process out of the manufacturing procedure, and the flatness of the image is also deteriorated as the inside surface curvature radius of the panel is decreased due to the high wedge.
Moreover, if the thickness of the peripheral portion of the panel 3 is increased, its transmission rate is noticeably reduced as well, extremely lowering the uniformity of brightness.
For instance, suppose that the wedge of the panel used in a 27-inch cathode ray tube is about 2.2. Then the transmission rate at the central portion of the panel is 51% while the transmission rate at the peripheral portion of the panel is about 25%, which is less than ½ (0.5) of the transmission rate at the central portion of the panel. This resultantly breaks down the uniformity of brightness, making the cathode ray tube totally useless. Therefore, the ratio (%) of transmission rates should be at least 59% or higher so as to allow the cathode ray tube to carry out its basic performance.
FIG. 4 is a diagram explaining the outside surface curvature of the panel, which is substantially flat.
As shown in FIG. 4, P(x, y, z) indicates a point on the substantially flat outside surface of the panel. The outside surface curvature radius of the panel can be expressed by the following mathematical formula I.
[Mathematical Formula I]
      Curvature    ⁢                  ⁢    radius    =                              (                                                    x                2                            +                              y                2                                              )                2            +              z        2                    2      ×      z      Given that the origin, coincident with the optical axis of the outside surface of the panel, is (0,0,0), the vector (x,y,z) indicates a distance from the origin to an arbitrary point on the outside surface, along the x-axis, y-axis, and z-axis.
The outside surface curvature radius of the panel with the substantially flat outside surface in the conventional cathode ray tube is approximately 100,000 mm. A strong point of this type of panel is that since a viewer perceives the panel as flat, the sense of flatness of the screen is secured and the viewer hardly sees distorted images.
On the other hand, when the wedge of the panel whose outside surface is substantially flat becomes high, e.g. greater than 2.0, the thickness of the diagonal end becomes extremely large and this affects the contrast of brightness of images.
The following mathematical formula II represents the transmission rate of the panel.
[Mathematical Formula II]Transmission rate (TM)=(1−Re)2×e−kt×100(%)
W\where Re denotes the reflectivity of glass; k denotes the absorbency index; and t denotes the thickness of glass.
The above formula shows that as the wedge of the panel is increased, the ratio of the thickness of a diagonal end to the thickness at a central portion of the panel becomes greater, and the difference between the transmission rate at the central portion of the panel and at the peripheral portion of the panel becomes larger. As a result, the brightness at the central portion and the peripheral portion will be much different from each other and cause visual discomfort to the viewer.
As an attempt to solve this problem, some used a panel having at least 85% of transmission rate at the central portion, hoping to secure the uniformity of brightness without deteriorating the peripheral transmission rate.
Table 1 shows contrast ratios (%) of the peripheral portion to the central portion and transmission rates (%) of the peripheral portion to the central portion, given an illumination with an external light of 200 lux (lx).
TABLE 1Transmission rateRatio of contrastRatio of transmission rate ofof centralof peripheral portionperipheral portion to centralportion (%)to central portion (%)portion (%)9014.098.78514.993.28016.087.87517.182.37018.476.86519.771.36021.165.95522.660.45024.254.9
In short, as the transmission rate of the central portion is improved, the ratio of the transmission rate of the peripheral portion to the transmission rate of the central portion is also improved but the ratio of the contrast of the peripheral portion to that of the central portion is lowered.
Although it is possible to secure uniformly bright images without reducing the transmission rate at the peripheral portion by using panel having a transmission rate at the central portion of the panel being higher than 85%, this also gives rise to other problems, e.g. excessive brightness or poor contrast characteristics.
Especially when the contrast is bad and the cathode ray tube is operated in a place where the illumination of the external light is greater than 200 lux (lx), it becomes virtually impossible to obviate the visual discomfort problem.
One solution to overcome the above drawback is putting a coating or film on the panel, however, the method was not found very favorable because it required extra processing and cost.